The present invention relates to a foam injection molding method wherein high-expansion ratio foaming is effected in a mold. More particularly, the present invention relates to a foam injection molding method wherein high-expansion ratio foaming is effected in a mold under control by expanding and contracting the cavity, while keeping it closed, by moving a mold element.
Plastics can be given various properties by foaming them. Particularly, the following various properties can be imparted to plastics by foaming: heat insulating, sound absorbing, vibration damping, buoyant, elastic, lightweight, liquid guiding, dust-resistant (filtering), friction and non-slip properties. Many foam moldings have recently been employed for various purposes by making use of these properties. It is expected that there will be an increase in the demand for composite moldings incorporating a foamed molded part in a part thereof to utilize the above-described properties.
At present, foaming is carried out mainly by injection molding and extrusion. Injection molding, which is superior to extrusion in moldability, is a method wherein a relatively small amount of molding material containing a blowing agent is injected into a cavity in a mold under low pressure, and the cavity is filled with the molding material by foaming caused by the blowing agent. As the blowing agent, a low-boiling point petroleum solvent may be used, but an azodicarbonamide or oxybissulfonyl hydrazide compound is generally employed.
Such a foam injection molding method has difficulty in controlling the amount of molding material injected because the molding material is foamed in the cavity with a fixed volumetric capacity to fill it. Accordingly, moldings produced by foaming are generally low-expansion ratio molded parts; no molded parts of high-expansion ratio can be obtained by the above-described injection molding method.
FIGS. 1(a) and 1(b) show a known high-expansion ratio injection molding method. As shown in FIG. 1(a), the conventional injection molding method employs an injection mold composed of two mold elements, i.e., a stationary mold element 01 and a movable mold element 02. The movable mold element 02 is slidable relative to the stationary mold element 01. As shown in FIG. 1(a), an initial cavity 03 is formed by the two mold elements 01 and 02. The initial cavity 03 is rapidly filled with a molding material containing a blowing agent. Immediately after the filling process, the movable mold element 02 is moved backward relative to the stationary mold element 01 to enlarge the cavity volume, thereby forming a final cavity 04 as shown in FIG. 1(b).
The charged molding material is foamed in the expanded cavity 04, formed as described above. This foam injection molding method enables the expansion ratio to be increased. In other words, the described foam injection molding method enables high-expansion ratio foaming.
If the mold is cooled immediately after the charging of the molding material, the material that is present at the interface between the cavity and the mold inner surface is immediately cooled. When the material at the interface is rapidly cooled, a skin is formed on the foaming material molded, and a relatively lightweight molded part having a skin layer is obtained. Such foam moldings, in which the interior is protected by the skin layer, can be utilized as products or materials which have various properties.
The above-described conventional method, in which the cavity is expanded by moving two mold elements relative to each other to effect high-expansion ratio foaming, suffers, however, from the following two problems: The interfaces a and b of the cavity expansion that is added to the initial cavity, that is, the cavity portion that remains when the initial cavity, which is shown in FIG. 1(a), is subtracted from the final cavity, which is shown in FIG. 1(b), are limited to surfaces S, as shown in FIG. 2, which are formed by scanning movement of the line L of intersection of the cavity forming surface A of the stationary mold element 01 and the cavity forming surface C of the movable mold element 02 having a sliding surface B that slides on the cavity forming surface A. Accordingly, the degree of freedom of the final shape that can be given to the foamed part is limited to a considerably low level.
More generally, when only two mold elements are employed, it is impossible to have a cavity interface that disables the initial cavity 03, which is formed by the two mold elements, from being shifted to the final cavity 04 by moving the two mold elements relative to each other while keeping the cavity closed.
A cavity interface with which the initial cavity 03 cannot be shifted to the final cavity 04 with the cavity kept closed is such that a portion of the cavity forming surface A in the vicinity of the line L of intersection of the two cavity forming surfaces A and C of the two mold elements 01 and 02, which form the closed cavity 03 when they are joined together, is formed from a surface that is not parallel to the direction of movement of the two mold elements 01 and 02.
Since the foam injection molding method that employs only two mold elements suffers from the above-described restriction, it is impossible to form three-dimensional molded parts of high-expansion ratio whose interfaces have neither parallel sliding surfaces nor a surface consisting of a set of parallel lines, for example, a spherical part having a spherical interface, an annular part, such as a doughnut-shaped part, which has a torus interface, a polyhedron, such as a regular tetrahedron, in which any two of the four interfaces are not parallel to each other, a conical part, or a complicated three-dimensional part, such as a tetrapod, which consists of a combination of a plurality of conical surfaces, although it is possible to form a three-dimensional object, e.g., a circular cylinder, which has an interface consisting of a set of parallel lines, by a high-expansion ratio injection molding process. This is the first problem of the conventional method.
Molded parts produced by high-expansion ratio foaming extremely vary in the physical properties according to the bubble size distribution and porosity. In the high-expansion ratio foaming process that is carried out by expanding the cavity, various factors, such as the pressure and temperature in the cavity, the viscosity of the molding material to be foamed, and the shape of the cavity, particularly have effects on the uniformity of dispersion of bubbles generated and the bubble size distribution characteristics. It is not easy to control the uniformity of dispersion of bubbles and the bubble size distribution characteristics by using only the ratio of the volume of the initial cavity to the volume of the final cavity and the cavity expanding time. This is the second problem of the conventional method.